The present invention relates to pressure casting lines for sanitary appliances, in particular lines for forming sanitary appliances with moulds comprising of only two parts.
Sanitary appliances are formed by casting slip in hygroscopic and/or permeable moulds, which can includes two or more arts depending on the complexity of the appliance to be formed.
Plaster moulds have been traditionally used, the absorbent power of the plaster being utilized to dehydrate the ceramic slip. However the use of this type of mould involves very lengthy forming cycles. Moreover plaster moulds last for only a small number of forming cycles.
For these reasons manufacturers have devised porous resin moulds possessing much better mechanical characteristics than plaster moulds, with the result that pressure casting techniques can be used, in which the slip is fed generally at a pressure of between 3 and 15 bar.
To counteract the forces generated by the pressurized slip in a direction perpendicular to the contact surfaces between the various mould parts, resin moulds have to be clamped by special sophisticated closure devices of high cost. For this purpose, machines are known which exert a mould clamping force, in the mould closure direction, which is sufficient to counteract the force exerted by the internal mould pressure at any time.
These moulds are stressed not only in their closure direction but also in directions perpendicular to the closure direction, hence they have to include lateral reinforcement structures the purpose of which is to limit their outward deformation. Such structures generally comprise adjustable means enabling the mould sides to be precompressed. This enables mould deformation to be reduced with a consequent increase in its strength, both because resins withstand compression better than tension, and because the deformation imposed on the mould, being opposite to that caused by the slip pressure, at least partly nullifies the effects, thereby limiting the tensions within the mould during casting.
Notwithstanding said adjustable means in the form of lateral reinforcements, the maximum usable casing pressure remains limited, in the known art, by the deformation which the mould undergoes as a result of the stresses to which it is subjected.
The deformation induced by the casting pressure also depends on the shape of the piece to be formed. In this respect, for certain types of appliances the mould is subjected, in certain parts, to very high stresses which, on the one hand can cause premature mould fracture and, on the other hand can produce considerable tensions within the appliance being formed. These tensions result in fracture of the appliance either during its removal from the mould or during one of the subsequent processing stages, i.e., drying and firing. Consequently, in forming the aforesaid types of appliances, the casting pressure used must not be too high, which increases the forming time.
To increase the casting pressure, manufacturers have provided increasingly rigid lateral containment structures, in order to limit the outward deformation of the mould. However, even if a theoretically (infinitely) rigid lateral containment structure were possible, it would not completely solve the problem because of tile compressibility of the resin with which the moulds are made. In this respect, even if outward deformation of the mould were completely prevented, the forming cavity would expert because of the casting pressure, thereby elastically contracting the resin forming the mould walls.
Elastic contraction of the mould walls results in substantial problems on removing the appliance from the mould, in that when the casting pressure ceases, the walls return to their original volume to consequently clamp the appliance within the forming cavity.
An object of the present invention is to overcome the aforesaid drawbacks within the frames cork of a rational, reliable arid relatively low-cost solution.
This and further objects are attained by providing an apparatus for pressure casting sanitary appliances by moulds comprising only two parts, namely a male part and a female part penetrated by this latter.
In the casting apparatus of the present invention, the moulds are positioned in series, and formed into blocks composed of the female part of one mould and that male part which is to be inserted into the female part of the adjacent block. When the male part of one block is inserted into the female part of the next block, the mould closure forces are compensated between one mould and the next along the forming line which has therefore to counteract only the closure force of a single mould, whatever the number of its constituent moulds. The line construction is therefore light and economical, practically without limitation on the number of moulds present therein.
Those forces directed perpendicular to the mould closure direction are compensated for, according to the present invention, by are external frame comprising means for exerting from the outside a pressure on the mould which is closed, substantially equivalent to that which the slip exerts from the inside, so as to prevent, at any moment, any deformation thereof.
The pressure exerted from the outside can be conveniently controlled to be above or below the casting pressure, to allow small deformations of a desired amount, to facilitate forming the piece and removing it from the mould.
The means for exerting a pressure on the outer lateral surface of the mould can be an element, which surrounds at least that lateral surface of the block corresponding to the forming cavity, and this element can be inflated by a suitable fluid.